Which Came First, the Galaxy or the Black Hole?

2000: Karl Gebhardt, Laura Ferrarese, David Merritt

At first glance, there doesn't seem to be much relationship between the size of a galaxy and the size of the supermassive black hole at its center. The black hole at the center our home galaxy, the Milky Way, for example, is only about one-tenth as massive as the one at the center of M31 even though the two galaxies are near twins.

But as you look deeper — deep into the neighborhoods that surround the black holes — a striking correlation emerges: There's a direct relationship between the mass of the black hole and the mass of the galaxy's central "bulge." This relationship was discovered in 2000 by Karl Gebhardt and, independently, by Laura Ferrarese and David Merritt. Other relations between the black hole and the host galaxy had been noted in the past, but no relation was as tight as this new discovery.

Astronomers have discovered dozens of supermassive black holes at the hearts of galaxies. The black holes appear either in spiral galaxies with big bulges of stars at their centers, like the Milky Way, or in galaxies that are nothing but bulges, with no surrounding disks, like the giant elliptical galaxy M87.

By measuring the masses of both the black holes and the bulges of stars, Gebhardt, Ferrarese and Merritt discovered that the mass of the black hole is always around 0.15 percent of the total mass of the bulge. The sample of black holes is large enough that it's unlikely that this relationship is just a coincidence. Instead, the birth of the black hole and the galaxy seem to be intimately related.

Since this discovery, astronomers have determined that the black hole probably plays a key role in forming the bulge.

The black hole's gravity pulls in surrounding gas clouds. As the clouds squeeze together they split into small clumps that give birth to stars. These stars and the remaining gas fall toward the black hole, where they form a large, bright, superhot disk around the black hole. Such disks power quasars, which are the most luminous objects in the universe.

Radiation and "winds" from the disk become so strong that they start to push away the remaining gas in the galaxy's bulge. Powerful jets of charged particles from the black hole's poles also sweep away the surrounding gas. These processes are so efficient that they clear most of the gas out of the bulge, leaving nothing to spawn new stars. But they also leave nothing to fuel the disk around the black hole, so it soon disappears as its gas is either swallowed by the black hole or blown back out into space. With no new material to feed it, the quasar goes dark.

When the process is complete the galaxy is left with a supermassive black hole at its center and a surrounding "bulge" of stars that's about 700 times the black hole's mass — a relationship that seems to hold up in every galaxy.

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